Add Seq<T> onEmptySwitch(Supplier<? extends Stream<? extends T>>)

[lukaseder]

As suggested by @johnmcclean-aol (#177 (comment)), an onEmptySwitch(Supplier<? extends Stream<? extends T>>) method might be useful in some situations.

Criticism:

• The current onEmptyXXX() names were inspired by Optional.orElseXXX(). We should think about the term switch, or if there is any more suitable term for this action
• Due to generic type erasure, we cannot overload a single onEmptySwitch(Supplier<E>) method with different bounds for <E>. What would be the best upper bound? Seq? Stream? Iterable?